Selena B215 FM front-end with TA7358 - UK Vintage Radio Repair and Restoration Discussion Forum

dan.mih · 24th Jan 2020, 8:40 pm

Hello,

I have found this link, which i translated from bulgarian with Google:

https://translate.google.com/transla...rziev_ukw.html

I noticed a difference between the datasheet and the schematic KN34PC built:

In the datasheet pin 3 goes to GND via half of the variabile capacitor, capacitor and coil. KN34PC connectes pin 3 to Vcc via variabile capacitor etc.

Is this design going to work ? I have assemebled almost all the parts. I have to build the coils and buy the 10,7Mhz transformer.

[img]************************ZWBXShXp/IMG-20200121-211546.jpg[/img]

orbanp1 · 24th Jan 2020, 11:03 pm

Actually, pin 3 goes to Vcc on the datasheet, and to the +1.6V - +6V line on the had drawn diagram, so both of them shows the same!

If you prefer, you can attach the frame of the variable cap to the ground, from AC point of view it is the same as the Vcc.

The B215-216 provides +4.2V for supply according to the schematics, that is well within the operating range of the IC.

What is missing is the AFC, you might want to add that to the circuit.

Peter

dan.mih · 25th Jan 2020, 8:32 pm

Thank you for your answer. I think i have done a mistake.
From the drawn schematic pin 8 goes to GND. In the datasheet, pin 8 goes to Vcc. This is what i wanted to point out.

Dickie · 25th Jan 2020, 9:14 pm

Pin 3 has to be returned to +Vcc via the tuning coil according to the data sheet, it is presumably one of the DC supplies to the ic. Assuming the two gang tuning capacitor has a common frame it means pin 8 must go to +Vcc also. This doesn't matter as far as the ic goes because of the 15pf series capacitor and the fact that +Vcc is close to RF ground anyway, as Peter says. The hand drawn cicuit will not work as it will short out the supply.

dan.mih · 26th Jan 2020, 11:54 am

„The hand drawn circuit will not work as it will short out the supply”.
On the other side, now i saw in the datasheet that one version for testing the IC is with pin to GND via the tuning circuit...
So probably, the hand drawn circuit has its logic...

Dickie · 26th Jan 2020, 1:08 pm

As Peter says, You can connect the frame of the tuning capacitor to ground or +Vcc, they are both RF ground if there is enough low inductance supply decoupling around. What you can't do is connect one half of the frame to ground and the other half to +Vcc, as the hand drawn circuit suggests. Unless you have a very odd two gang capacitor, that is.

dan.mih · 31st Jan 2020, 7:10 pm

I have built the circuit it works. I am almost satisfied with the results.

I have noticed that if i span L2 like a fan (i spread the turns), the reception gets a little stronger. Is this normal? As far as i know, if you spread the coil the inductance decreases. I had the impression that high frequency coils need higher inductance.

The coils do not match exactely the datasheet.

L2 - 4 turns, 0,8mm enamelled copper, 4,5mm diameter (in datashet: 2,75 turns, 0,5mm, enamelled copper dorn, 4,5mm diameter, ferrite core)

LO - 2,75 turns, 0,6mm enamelled copper, on 6mm inductor with ferite core (in datasheet: 2,5 turns, 0,5mm enamelled copper, 4,5mm diameter, ferrite core).

[img]************************9DcMcwSH/100-5793.jpg[/img]

[img]************************kBQGrvTy/100-5801.jpg[/img]

Now my questions are:
1. Are the wire diameter and coil diamter critical? As far as i have read, in those circuits, thse values are relative because the current is very low.

2. Does silver plated copper have any advantage for this circuit? I have 0,8mm silver plated copper.

orbanp1 · 1st Feb 2020, 12:51 pm

Hi Dan,

Coil diameter does matter, the inductance of the coil, which is important, does depend on it.
Do look for a coil inductance calculator on the web, it will demonstrate this!
At that high frequencies there is "skin effect" (https://en.wikipedia.org/wiki/Skin_effect) so a bigger wire diameter will give more conducting surface area, less resistance for the high frequency, and consequently higher Q for the coil. Silver plating also increases Q because of less resistance.
In practice for FM front-ends "regular" wires seem adequate.

Do you have a link to the article and the PCB templates?

Regards, Peter

dan.mih · 1st Feb 2020, 8:10 pm

Thank you for the answer.
The download link is at the bottom of this page:

https://translate.google.com/transla...rziev_ukw.html

dan.mih · 1st Feb 2020, 8:21 pm

I don t know what i have done wrong. I filed down the PCB corners to fit the box, trimmed the leads of the capacitors and it s not working anymore. It is only working if i touch with a screwdriver pin 4. Which is hilarious and annoying...

https://www.youtube.com/watch?v=HTo6...ature=youtu.be

orbanp1 · 2nd Feb 2020, 12:50 am

Hi Dan,

Thanks for the link!

Double check everything!
Do look for cold solder joints, retouch every soldering, and also see if any other mechanical force like bending, pushing, on the IC, or on every other component, does make a difference.
Do replace the IC if nothing hepls, it is possible that the IC developed some internal intermittent connections that the touching/bending overcomes.

Regards, Peter

24th Jan 2020, 8:40 pm	#1
dan.mih Triode Join Date: Aug 2019 Location: Toplita, Romania Posts: 11	Selena B215 FM front-end with TA7358 Hello, I have found this link, which i translated from bulgarian with Google: https://translate.google.com/transla...rziev_ukw.html I noticed a difference between the datasheet and the schematic KN34PC built: In the datasheet pin 3 goes to GND via half of the variabile capacitor, capacitor and coil. KN34PC connectes pin 3 to Vcc via variabile capacitor etc. Is this design going to work ? I have assemebled almost all the parts. I have to build the coils and buy the 10,7Mhz transformer. [img]************************ZWBXShXp/IMG-20200121-211546.jpg[/img] Attached Thumbnails

24th Jan 2020, 11:03 pm	#2
orbanp1 Heptode Join Date: Nov 2009 Location: London, Ontario, Canada Posts: 675	Re: Selena B215 FM front-end with TA7358 Actually, pin 3 goes to Vcc on the datasheet, and to the +1.6V - +6V line on the had drawn diagram, so both of them shows the same! If you prefer, you can attach the frame of the variable cap to the ground, from AC point of view it is the same as the Vcc. The B215-216 provides +4.2V for supply according to the schematics, that is well within the operating range of the IC. What is missing is the AFC, you might want to add that to the circuit. Peter

25th Jan 2020, 8:32 pm	#3
dan.mih Triode Join Date: Aug 2019 Location: Toplita, Romania Posts: 11	Re: Selena B215 FM front-end with TA7358 Thank you for your answer. I think i have done a mistake. From the drawn schematic pin 8 goes to GND. In the datasheet, pin 8 goes to Vcc. This is what i wanted to point out. Attached Thumbnails Last edited by dan.mih; 25th Jan 2020 at 8:38 pm.

25th Jan 2020, 9:14 pm	#4
Dickie Octode Join Date: Sep 2004 Location: St. Albans, Hertfordshire, UK. Posts: 1,478	Re: Selena B215 FM front-end with TA7358 Pin 3 has to be returned to +Vcc via the tuning coil according to the data sheet, it is presumably one of the DC supplies to the ic. Assuming the two gang tuning capacitor has a common frame it means pin 8 must go to +Vcc also. This doesn't matter as far as the ic goes because of the 15pf series capacitor and the fact that +Vcc is close to RF ground anyway, as Peter says. The hand drawn cicuit will not work as it will short out the supply. __________________ Regards, Richard, BVWS member

26th Jan 2020, 1:08 pm	#6
Dickie Octode Join Date: Sep 2004 Location: St. Albans, Hertfordshire, UK. Posts: 1,478	Re: Selena B215 FM front-end with TA7358 As Peter says, You can connect the frame of the tuning capacitor to ground or +Vcc, they are both RF ground if there is enough low inductance supply decoupling around. What you can't do is connect one half of the frame to ground and the other half to +Vcc, as the hand drawn circuit suggests. Unless you have a very odd two gang capacitor, that is. __________________ Regards, Richard, BVWS member

31st Jan 2020, 7:10 pm	#7
dan.mih Triode Join Date: Aug 2019 Location: Toplita, Romania Posts: 11	Re: Selena B215 FM front-end with TA7358 I have built the circuit it works. I am almost satisfied with the results. I have noticed that if i span L2 like a fan (i spread the turns), the reception gets a little stronger. Is this normal? As far as i know, if you spread the coil the inductance decreases. I had the impression that high frequency coils need higher inductance. The coils do not match exactely the datasheet. L2 - 4 turns, 0,8mm enamelled copper, 4,5mm diameter (in datashet: 2,75 turns, 0,5mm, enamelled copper dorn, 4,5mm diameter, ferrite core) LO - 2,75 turns, 0,6mm enamelled copper, on 6mm inductor with ferite core (in datasheet: 2,5 turns, 0,5mm enamelled copper, 4,5mm diameter, ferrite core). [img]**********************9DcMcwSH/100-5793.jpg[/img] [img]*********************kBQGrvTy/100-5801.jpg[/img] Now my questions are: 1. Are the wire diameter and coil diamter critical? As far as i have read, in those circuits, thse values are relative because the current is very low. 2. Does silver plated copper have any advantage for this circuit? I have 0,8mm silver plated copper. Attached Thumbnails Last edited by dan.mih; 31st Jan 2020 at 7:28 pm.*

1st Feb 2020, 12:51 pm	#8
orbanp1 Heptode Join Date: Nov 2009 Location: London, Ontario, Canada Posts: 675	Re: Selena B215 FM front-end with TA7358 Hi Dan, Coil diameter does matter, the inductance of the coil, which is important, does depend on it. Do look for a coil inductance calculator on the web, it will demonstrate this! At that high frequencies there is "skin effect" (https://en.wikipedia.org/wiki/Skin_effect) so a bigger wire diameter will give more conducting surface area, less resistance for the high frequency, and consequently higher Q for the coil. Silver plating also increases Q because of less resistance. In practice for FM front-ends "regular" wires seem adequate. Do you have a link to the article and the PCB templates? Regards, Peter

1st Feb 2020, 8:10 pm	#9
dan.mih Triode Join Date: Aug 2019 Location: Toplita, Romania Posts: 11	Re: Selena B215 FM front-end with TA7358 Thank you for the answer. The download link is at the bottom of this page: https://translate.google.com/transla...rziev_ukw.html

1st Feb 2020, 8:21 pm	#10
dan.mih Triode Join Date: Aug 2019 Location: Toplita, Romania Posts: 11	Re: Selena B215 FM front-end with TA7358 I don t know what i have done wrong. I filed down the PCB corners to fit the box, trimmed the leads of the capacitors and it s not working anymore. It is only working if i touch with a screwdriver pin 4. Which is hilarious and annoying... https://www.youtube.com/watch?v=HTo6...ature=youtu.be

2nd Feb 2020, 12:50 am	#11
orbanp1 Heptode Join Date: Nov 2009 Location: London, Ontario, Canada Posts: 675	Re: Selena B215 FM front-end with TA7358 Hi Dan, Thanks for the link! Double check everything! Do look for cold solder joints, retouch every soldering, and also see if any other mechanical force like bending, pushing, on the IC, or on every other component, does make a difference. Do replace the IC if nothing hepls, it is possible that the IC developed some internal intermittent connections that the touching/bending overcomes. Regards, Peter